Quantification of neocortical ratios in stem primates.

Area of Science:

• Paleoneurology
• Primate Evolution
• Comparative Anatomy

Background:

• Extant euprimates (crown primates) exhibit a disproportionately large neocortex compared to other mammals.
• The evolutionary timing of this neocortical expansion remains unclear.
• Previous studies on Eocene euprimate fossils suggested significant neocortical expansion was already present.

Purpose of the Study:

• To investigate neocorticalization in stem primates (plesiadapiforms).
• To determine if relative neocortical expansion was unique to crown primates or present earlier in primate evolution.
• To provide quantitative data on brain evolution in early fossil mammals.

Main Methods:

• Calculated ratios of neocortex to endocast surface area for plesiadapiforms.
• Utilized virtual endocasts of fossil specimens: Ignacius graybullianus (paromomyid) and Microsyops annectens (microsyopid).
• Compared plesiadapiform data with published estimates for Plesiadapis tricuspidens and general archaic mammal data.

Main Results:

• Plesiadapiform neocorticalization ratios were comparable to archaic mammals generally.
• These ratios fell within the 95% confidence intervals for archaic mammals, contrasting with early Tertiary euprimates.
• Data for Ignacius graybullianus and Microsyops annectens were similar to Plesiadapis tricuspidens.

Conclusions:

• Relative neocortical expansion may not be uniquely characteristic of the crown primate radiation.
• A shift in brain organization, potentially linked to visual system development, likely occurred in the earliest euprimates.
• Further research is needed due to limitations including fossil sample size and potential allometric effects.